Tomographic imaging methods are distinguished by virtue of the fact that internal structures of an examination object can be examined without, in the process, having to carry out invasive interventions on the latter. A possible type of tomographic image generation consists of recording a number of projections of the object to be examined from different angles. A two-dimensional slice image or a three-dimensional volume image of the examination object can be calculated from these projections.
Computed tomography is an example of such a tomographic imaging method. Various methods for scanning an examination object using a CT system are known. By way of example, circular scans, sequential circular scans with feed or spiral scans are applied. Other types of scans, which are not based on circular movements, are also possible, such as e.g. scans with linear segments. Absorption data of the examination object are recorded from different recording angles with the aid of at least one x-ray source and at least one opposing detector and the absorption data or projections collected thus are calculated by means of appropriate reconstruction methods to form slice images through the examination object.
In order to reconstruct computed tomographic images from x-ray CT data sets of a computed tomography instrument (CT instrument), i.e. from the detected projections, a so-called filtered back projection (FBP) is used these days as standard method. After the data capture, a so-called “rebinning” step is usually carried out, during which the data generated by the beam propagating in a fan-shaped manner from the source are reordered such that they are available in a form as if the detector were hit by x-rays running towards the detector in parallel. The data are then transformed into the frequency space. Filtering takes place in the frequency space and the filtered data are subsequently transformed back. There then is a back projection onto the individual voxels within the region of interest with the aid of the thus re-sorted and filtered data.
The FBP method belongs to the group of approximate reconstruction methods. Furthermore, there is the group of exact reconstruction methods, but these are currently barely used. Finally, iterative methods form a third group of reconstruction methods.